Re: [Xen-devel] domain creation vs querying free memory (xend and xl)

[Dario Faggioli <raistlin@xxxxxxxx>]

On Thu, 2012-09-27 at 19:24 +0400, George Shuklin wrote: 
> not sure about xl/xm, but xapi performs one start at time, so there is 
> no race between domains for memory or other resources.
> 
IIRC, xl has a vary coarse grain locking mechanism in place for domain
creation too. As a result of that, you shouldn't be able to create two
domains at the same time, which should be enough for preventing the
situation described in the original e-mail for occurring.

Looking at acquire_lock() and release_lock() (and at where they are
called) in xl code should clarify whether or not that is enough to
actually avoid the race (which I think it is, but I might be wrong :-D).

That being said, there still is the room for races, although not wrt
domain creation, as, for instance, there isn't any synchronization
between creation and ballooning, which both manipulate memory. So, maybe
thinking about some kind of reservation-based/transactional mechanism at
some level might make actual sense.

Unfortunately, I've no idea about how xm works in that respect.

Hope this at least help clarifying the situation a bit. :-)

Thanks and Regards,
Dario

-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
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Dario Faggioli, Ph.D, http://retis.sssup.it/people/faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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